For other uses, see Vein (disambiguation).
In the circulatory system, a vein is a blood vessel that carries blood toward the heart. The vessels that carry blood away from the heart are known as arteries. The study of veins and diseases of the veins is known as phlebology and is a poorly defined discipline that is being developed through a variety of interdisciplinary scientists rather than having a specific set of specialists.When veins are pumped too hard with blood, they go through a rough period called the plamsic veteriority. This causes the veins to swell and eventually explode.
Cross section of a vein showing a valve which prevents backflow
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Contents
- 1 Function
- 2 Anatomy
- 3 Common diseases
- 4 Notable veins and vein systems
- 4.1 List of important named veins
- 4.2 Names of important venule systems
- 5 Medical interest
- 6 Color
- 7 See also
- 8 External Links
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Function
Veins serve to return blood from organs to the heart. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries, entering the veins filled with cellular waste and carbon dioxide. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped to the pulmonary arteries and eventually lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.
The return of blood to the heart is assisted by the action of the skeletal-muscle pump which helps maintain the extremely low blood pressure of the venous system.
Anatomy
Most veins have one-way valves called venous valves to prevent backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. The hollow internal cavity in which the blood flows is called the lumen. Veins are surrounded by helical bands of smooth muscles which help maintain blood flow to the right atrium. The total capacity of the veins is more than sufficient to hold the entire blood volume of the body; this capacity is reduced through the constriction of smooth muscles, minimizing the cross-sectional area (and hence volume) of the individual veins and therefore total venous system.
Fainting can be caused by failure of the skeletal-muscular pump. Long periods of standing can result in blood pooling in the legs, with blood pressure too low to ascend to the heart. Neurogenic and Hypovolaemic shock can also cause fainting. In these cases the smooth muscles surrounding the veins become slack and fill with blood, absorbing a large portion of the total blood volume, keeping blood away from the brain and causing unconsciousness.
Common diseases
Common diseases of the veins include varicose veins (treated through radio-frequency ablation, lasers and vein stripping) and deep vein thrombosis.
Notable veins and vein systems
The pulmonary veins carry relatively oxygenated blood from the lungs to the heart. The superior and inferior vena cavae carry relatively deoxygenated blood from the upper and lower systemic circulations, respectively.
A portal venous system is a series of veins or venules that directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system.
List of important named veins
- Pulmonary veins
- Portal vein
- Superior vena cava
- Inferior vena cava
- Femoral vein
- Great saphenous vein
- Jugular veins
Names of important venule systems
- Portal venous system
- Pulmonary venous system
- Systemic venous system
Medical interest
Venous valves prevent reverse blood flow.
Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes, nutrition, and medications. The latter is called intravenous delivery. It can be done by an injection with a syringe, or by inserting a catheter (a flexible tube). In contrast to arterial blood which is uniform throughout the body, the blood removed from veins for testing can vary in its contents depending on the part of the body the vein drains. In example, blood drained from a working muscle will contain significantly less oxygen and glucose than blood drained from the liver. However the more blood from different veins mixes as it returns to the heart, the more homogeneous it becomes.
If an intravenous catheter has to be inserted, for most purposes this is done into a peripheral vein (a vein near the surface of the skin in the hand or arm, or less desirably, the leg). Some highly concentrated fluids or irritating medications must flow into the large central veins, which are sometimes used when peripheral access cannot be obtained. Catheters can be threaded into the superior vena cava for these uses: if long term use is thought to be needed, a more permanent access point can be inserted surgically.
The precise location of veins is much more variable from person to person than that of arteries.
Color
The blood carried by veins is dark red due to its high percentage of CO2 as it returns to the heart (in contrast to the high levels of O2 in arterial blood, which is bright red). Veins appear blue because the subcutaneous fat in the skin absorbs lower-frequency light, permitting only the highly energetic blue wavelengths to penetrate and reflect off human skin.
